1. Field of the Invention
The present invention relates to a device and a method for moulding and vulcanizing a tyre, said moulding and vulcanizing operations being carried out at controlled volume.
2. Description of the Related Art
Traditionally, tyre manufacture involves the preparation, on a drum, of a green product consisting of a toroidal sleeve comprising a carcass that has a crown zone inserted between a pair of axially-opposed sidewalls, ending in beads for anchoring the tyre to a corresponding assembling rim, a belt structure and a tread band. The carcass and the belt structure are formed from preselected elastomeric materials and reinforcing elements, the tread band and the sidewalls are formed from strips of elastomeric material.
Then, the green tyre is extracted from the drum and submitted to vulcanization in a mould, under the combined action of heat and pressure.
Generally, the vulcanizing mould comprises a crown of radially adjustable sectors, interposed between two axially-opposed, mirror-finish cheeks, and an inflatable internal elastic chamber. Inside the mould, the green tyre is heated simultaneously from the outside and from the inside by means of a fluid under pressure (typically steam or hot water) circulating in the mould and introduced into the internal elastic chamber. During mould closure the cheeks are axially brought closer each other and the sectors are moved centripetally, to generate an outwardly closed cavity. During vulcanization, the elastic chamber expands and compresses the tyre from the inside against the mould. The tyre thus acquires the final tread pattern and the final shape.
During this moulding and vulcanization process, any variations of volume and/or of thickness of the green tyre can be tolerated because the internal elastic chamber, being deformable, automatically adapts itself to said variations and can “make adjustments” for them.
More recently, this traditional technique has been supplemented with a tyre moulding and vulcanization technique with a constant-volume mould. This consists of placing a predetermined volume of reinforcing elements and elastomeric material, which constitute the green tyre that is to be built and vulcanized, onto a rigid support of a toroidal shape (with outside surface matched to the inside surface of the finished tyre). The vulcanizing mould has an outer shell formed of a crown of sectors and of cheeks, and an inner part formed of the rigid support carrying the green tyre. When the mould is closed, the outer part of the mould is in its stop position and between said position and the rigid support there is the formation of a hollow space whose volume always remains constant.
The green tyre that is to be moulded and vulcanized in said mould must have a similar shape to that of the hollow space and a volume substantially equal to said constant volume of the hollow space of the mould because the moulding pressure depends on the difference between the volume of the green tyre and the volume of the hollow space. If the volume of the green tyre is less than that of the hollow space, the value of the moulding pressure is insufficient and hence the final result is unsatisfactory. On the other hand, in the case of excessive volume of the green tyre (in view of the constant volume available inside the mould and of the incompressibility of the rubber), high pressures are generated inside the mould. Said high pressures can generate excessive stresses on the mechanical components of the mould and lead to the deformation of the mould and hence to the production of a “distorted” product, or to the formation of unacceptable fins, if the elastomeric material flows into the clearances between said mechanical components and/or into suitable vents provided in the mould, or even to mould breakage.
Therefore, in moulding at constant volume, the volume of the green tyre should be substantially equal to the free volume of the closed mould. In practice, the volume of the green tyre must be just slightly greater than that of the hollow space in which it is accommodated.
EP-A0 264 600 describes a method and an equipment for moulding and vulcanizing a pneumatic tyre, at constant volume, in which elastomeric products and reinforcing elements are placed, in succession, on a rigid support, so that a green tyre is built up progressively. Placing of the elastomeric products is effected by means of at least one volumetric extruder having a discharge orifice positioned, relative to the support, in a lane in which the elastomeric product is applied for making the green tyre. The quantity of elastomeric product deposited is controlled by extruding a given volume from the volumetric extruder in relation to the speed of rotation of the support and to the deposition radius. The profile of the elastomeric product to be produced is obtained by means of a substantially meridian movement of the orifice relative to the rotating deposition surface.
The method and the equipment described in this document employ a mould in which the inside is constituted of a rigid support on which the green tyre is built. Thus, it is a constant-volume mould.
Therefore, in this case too, the method and the equipment described in the above-mentioned document require a green tyre that is to be moulded and vulcanized with a volume as constant as possible, and as close as possible to the theoretical (nominal) volume of the hollow space of the closed mould.
However, because of the inevitable variations of an industrial process, the quantity of elastomeric compound delivered is liable to fluctuations and the volume of the green tyres is not constant, whereas the volume of the hollow space of the mould is constant. Therefore, if there is a shortage of the elastomeric compound, constancy and correct moulding pressure are not guaranteed, whereas if there is excess of the elastomeric compound there are the problems and the difficulties described above.
Moreover, the above-mentioned variations in volume of the delivered material may depend, as well as on purely mechanical causes, also on the various characteristics of the elastomeric material and/or of the additives from one supplier to another or from batch to batch and/or on the process for mixing the components of the elastomeric compouds.
EP-A-0 400 496 describes a volumetric pump for the delivery of green rubber for an apparatus for moulding and vulcanizing a tyre, at constant volume. The pump comprises at least one delivery piston sliding in a cylinder with alternating motion between a top dead point and a bottom dead point. The pump also comprises a feed chamber, means for forced feed for feeding said cylinder and a discharge opening provided with a non-return valve. The cylinder wall comprises, as sole communication of the cylinder with the exterior, one or more inlet ports in which the feed chamber ends. The inlet port is arranged axially between the top dead point and the bottom dead point and is closed by the delivery piston during its movement towards the top dead point. The volume extruded in one cycle of the pump depends on the volume swept by the delivery piston between the point at which it closes the inlet port and the top dead point and by the fact that the feeding means and said delivery piston are actuated by a single input shaft.
This is a very complex pump which, however, does not guarantee deliveries of perfectly constant volumes.
The inventors observed that, in practice, in the process of building of a green tyre there is always a variation in the mass of materials delivered equal to ±δmass and hence a variability in the volume of the green tyre equal to ±δvolume relative to the nominal volume V0. For example, if ±δmass is of the order of ±100 g, assuming a specific density of 1.1 g/cm3, ±δvolume is equal to ±91 cm3. The variability of tyre volume, in percentage terms, is ±1%.
This value is, however, unacceptable for modem, sophisticated tyres intended for equipping high-performance cars.